A. Field of the Invention
This invention relates generally to electronic overfill protection devices for storage or transport tanks, and more particularly to a method and apparatus for performing diagnostic testing of the operation and electrical wiring integrity of the overfill protection devices.
B. Description of the Related Art
Tanks used for storing or transporting flammable fluids such as gasoline, diesel fuel and other hazardous petroleum products are often equipped with overfill protection devices to protect the environment.
The tanks can be mounted on tanker trucks or located underground at service stations. Tanker trucks are typically filled with the fluids using pumping equipment at loading racks of marketing terminals, and underground storage tanks are typically gravity filled from the trucks. An overfill protection device is used with each tank to disable the pumping equipment at the marketing terminals or close a truck-mounted flow valve at the service station when the limit of the tank's capacity is reached.
The overfill protection device typically has a detection circuit and a disable circuit. One or more probes, each located within a tank, are connected to the detection circuit over a separate channel. Each probe generates an electrical sensor signal that indicates when the fluid within the tank exceeds a pre-determined level. The detection circuit has a controller mounted near the tank (e.g., on the tanker truck or at the service station) that controls the operation of the probes. The detection and disable circuits are interconnected by a suitable electrical cable. In response to the sensor signal indicating that a particular tank is full, the disable circuit operates to stop flow into that tank (e.g., by disabling the pumping equipment at the loading rack or by closing the flow valve on the truck).
For applications involving filling of tanks with hazardous materials, the overfill protection device should be designed and operated in an "intrinsically safe" manner, and preferably be "fail safe." As defined in applicable standards promulgated by various governmental agencies and industrial organizations, an intrinsically safe circuit cannot produce any spark or thermal effect, either normally or in any likely fault condition, which is capable of causing the ignition of a mixture of the flammable fluid and its vapor or other combustible material in air.
"Fail safe" means that no single failure of a component or wiring in the overfill protection device will cause overfilling of the tank. From a practical standpoint, fail safe operation requires that designers of overfill protection devices anticipate failure modes for the devices, and devise preventive measures or build in redundancy of the components or wiring that are likely to fail.
While conventional intrinsically safe, fail-safe ("IS-FS") overfill protection devices are generally suitable for their intended applications, they do have drawbacks. Conventional IS-FS overfill protection devices generally lack any sophisticated diagnostic testing capability of validating that the probes are correctly wired and functional as intended. A conventional IS-FS overfill protection device is typically tested at most for a short circuit between the probe and ground, for an "open" connection to the probe, and to confirm the presence of an output signal from the probe. Such tests are performed under the normal operating excitations for the probes and use simple, conventional comparator logic.
Such testing by a conventional IS-FS overfill protection device cannot detect many common probe wiring problems, including those arising from corrosion and aging effects on wiring or probe-connections, accidental mis-wiring of probes, and so-called "cheating". Cheating is tampering with probe wiring to defeat the overfill protection normally provided by the device (for example, by connecting one working probe to two channels or more). (Because repair of a defective probe may require removal of a tank truck from service, a tank truck operator may wrongly resort to cheating if a probe becomes faulty and will not allow filling of the tank truck on which it is mounted.) Moreover, the problems of corrosion and aging effects, accidental mis-wiring of probes, and so-called "cheating" can not always be easily discovered, even by observant station attendants.
It would be desirable to provide an overfill protection device that can readily test for each of the above mentioned common problems and others that can affect the operation of such a device and the safety of transported or stored flammable fluids and other combustible material.
It would also be desirable to provide an overfill protection device that is configurable for the number of active probes which will be used in the field with the unit.